Many applications of pneumatic conveying have been proposed in the past. All sorts of materials have been tested for their ability to be pneumatically conveyed. It has been of course natural that most pneumatic conveyors typically involve soft and unabrasive materials and especially those having low density and small particle size. Almost all research work in this field relates to such materials mostly utilizing air as a low cost carrier gas at ambient temperature.
The feasibility of pneumatically conveying DRI pellets and other large sized iron-bearing particles at low and high temperatures with compatible carrier gases has been demonstrated by applicant's assignee and the advantages that it provides in steel making have been very favorably evaluated and published in the relevant technical literature. (See, for example, Iron and Steelmaker, Vol. 20, No. 7, July 1993, pp. 44-48 and Metallurgical Plant and Technology, Vol. 15, No. 4, August 1992, pp. 42-46). There is however a need for an automatic control system for running the pneumatic conveyor while providing protection to the equipment involved.
In U.S. Pat. No. 5,296,015 to Becerra, et al. assigned to the same assignee as this application, a pneumatic transport for DRI is disclosed. Becerra et al. disclose in one of the embodiments of their invention a pneumatic transport system in combination with a direct reduction reactor and an electric arc furnace wherein the carrier gas is recycled in a closed transport loop comprising a compressor, a feeder to introduce the DRI into the pneumatic conveying pipe, a set of gas disengaging means, for example a pair of sealable bins or cyclones and means for withdrawing carrier gas from the carrier gas loop.
Applicant's assignee in continuation-in-part U.S. patent application Ser. No. 08/071,756, filed Jun. 9, 1993 (the contents of which are incorporated herein by reference) at page 12 discusses in detail the difference between dilute and dense phase pneumatic transport of DRI (briefly stated, dilute phase flow is fluidized pneumatic flow of solid particles in carrier gas while dense phase flow involves discrete plugs of particles or at least wave-like shifting dunes of particles moved by the carrier gas).
In prior art applications of pneumatic transport (see, for example, Hyl Report, The Direct Reduction Quarterly, Vol. VII, No. 1, Spring 1993, pp. 4-8), the material to be transported is collected in a pressurizable bin. Then, after regulated pressurization the material is discharged from the bin at a feed point to the transport pipe. A carrier gas is fed to the bin at high pressure, so that the pressure drop from the feed point to the end of the conveyor, normally discharging at atmospheric pressure, is used for pushing the solids through the conveyor. If the solids demand more pressure difference along the transport pipe, with pressure isolation of the pneumatic conveyor from the preceding and following systems, there is no problem in increasing such pressure at the bin up to a level that the solid particles are forcibly moved through the pneumatic conveyor by means of the pressure of the carrier gas.
Most of the prior art applications of pneumatic conveying of solids have been made using air at ambient temperature and without any restriction for increasing the pressure level at the beginning of the transport line. Such systems do not control kinetic energy of the carrier gas, but instead control pressure differentials along the length of the pneumatic transport pipe.
Direct reduction processes have been widely installed and are currently operated in many countries in connection with so called "mini mills" for iron and steel making.
One of the problems in utilizing a pneumatic transport system directly and openly connected to a reduction reactor is to maintain both the bottom portion of the reactor and the initial portion of the conveying line at the same pressure. This is necessary, because if these pressures are different, then the carrier gas would enter the reactor, or conversely the greater pressure of reducing gas would create a loss-flow from the reactor to the conveying line, undesirably. If carrier gas should enter the reactor, the normal reduction process therein would be disturbed especially if the carrier gas should have a different composition than the process gas in the reactor. This should mean also that in the case of a reactor directly and openly connected to the transport line, the maximum length of the transport pipe was limited to the distance along which the solids could be transported by the pressure difference between the level of pressure of the reactor and the atmospheric pressure at the discharge point of the transport pipe (unless an additional pressurizable bin and supplemental carrier gas injection were to be inserted between the reactor discharge and the transport line).
The present invention provides a method and an apparatus for controlling the pneumatic transport of hot and cold DRI particles through a transport pipe in direct open connection with the DRI reactor without disturbing the reduction process and at a velocity sufficiently low so as to minimize the degradation in particle size of a friable solid material such as DRI. This invention also permits lower operational costs, because it minimizes the amount of carrier gas which must be vented, and because the DRI particles are transported at the minimum allowable velocity so that fines generation and particle breakage are minimized.